1. Field of the Invention
This invention relates to a wheel cover for automobiles, and more particularly to a wheel cover having good mechanical properties and an excellent surface appearance.
2. Description of Related Art
Automobile wheels are frequently covered with wheel covers. The purpose, among others, of such wheel covers is to protect the bolts and to provide a pleasing surface design.
Wheel covers can be made of polypropylene resin containing fillers and/or reinforcing material or agents. A polypropylene resin composition, for example, can contain a surface treated inorganic filler as shown in Japanese laid open patent No. Sho 60-47043. A propylene ethylene block copolymer composition can contain an inorganic filler which is treated with silane as shown in Japanese laid open patent No. Sho 61-197650. A crystalline propylene ethylene block copolymer composition can contain an inorganic filler prepared by pulverizing with a surface treating agent as shown in Japanese laid open patent No. Sho 62-235350. Another composition is ethylene propylene butene-1 copolymer rubber as shown in Japanese laid open patent No. Hei 2-296856. Also, ethylene propylene block copolymer composition can contain an inorganic filler and elastomer as shown in Japanese laid open patent No. Hei 5-98122. Further, a wheel cover can be molded of a propylene resin composition including glass fiber as shown in Japanese laid open patent No. Hei 6-80839.
These compositions exhibit improved thermal resistance and impact resistance due to the added filler or reinforcing agents.
However, even if these compositions are used in molding wheel covers, protrusions or raised marks can occur where the molding material, flowing from two different directions in the mold, meet and become joined together.
This type of flow of molding is shown in FIG. 4A. The wheel cover being made is formed by injecting a molding resin 95 into a cavity 72 of a mold 71 with flow from two different directions merging together, as shown. Frequently such cavities 72 have complicated shapes to mold the wheel cover as shown in FIG. 5. Therefore, movement of the stream 90 of resin 95 within the cavity 72 can be complicated resulting in a series of such junctions of counter flowing streams of resin.
As shown in FIG. 4B, a welded part or joint related protrusion 93 can occur at the place or joint where the counter flowing resin streams 90 meet and flow together. Several such joints 93 are shown in FIG. 5.
Numeral 94 in FIG. 5 designates the location of an injection gate for the wheel cover.
As is also shown in FIG. 4B, it is thought that the protrusions occur due to a shrinkage difference between the joint portion 91, where the glass fiber filler is orientated across the joint, or in a thickness direction, relative to adjacent joint areas 92 where the glass fibers are more orientated in the stream or flow direction.
The joint protrusion 93 that is formed is especially high when using reinforced polypropylene with glass fiber 82. Because of its height, the surface appearance of wheel cover is not desirable. When a conventional polypropylene compound is used, the protrusion height .DELTA.H is between 20 and 60 .mu.m.
As shown in FIG. 5, it is desirable to mold an improved design for wheel covers with various types of shapes which can include complicated structures or shapes including small or large holes 96. The surface appearance of the welded portion is remarkable.
Also, the possibility of using an isotropic filler was considered but, while the protrusion height is reduced, the mechanical characteristics of the compound are decreased.